Summation

\displaystyle\sum_{r=1}^n 3^r

What is the standard result for this?

Original post by will'o'wisp

\displaystyle\sum_{r=1}^n 3^r

What is the standard result for this?

It's a geometric progression, whose sum you can look up.

It's a geometric progression, whose sum you can look up.

Really? where d=can you find this? i couldn't find anything specific

sum up 3^r beginning with r=1 for n terms

sum up 3^r beginning with r=1 for n terms

thanks

Original post by will'o'wisp

Really? where d=can you find this? i couldn't find anything specific

Have you come across the idea of a GP and its sum before? If so, this is a standard result. To do it longhand, start with:

S=3+3^2+3^3+ \cdots + 3^n

then form

3S

, subtract this from

S

and rearrange to get

S

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